Harper D S, Jahn C L
Department of Biological Sciences, University of Illinois, Chicago 60680.
Proc Natl Acad Sci U S A. 1989 May;86(9):3252-6. doi: 10.1073/pnas.86.9.3252.
Sequence analysis of genes in four species of ciliated protozoa and analysis of tRNAs in Tetrahymena has demonstrated that TAG and TAA encode glutamine or glutamic acid in these organisms and TGA is the only stop codon. Thus, it has generally been assumed that all ciliates use a nonuniversal genetic code in which TGA acts as the sole termination codon. We have sequenced the linear DNA molecules that carry an actin gene and a beta-tubulin gene from the ciliate Euplotes crassus. These genes are shown to use TAA as a termination codon based on homology to known actin and beta-tubulin gene sequences. In addition, we have sequenced a portion of the 3' terminus of the E. crassus H4 histone gene and show that it also uses TAA as a termination codon. These data indicate that the timing of genetic code changes in the ciliates must be reconsidered.
对四种纤毛原生动物的基因进行序列分析以及对嗜热四膜虫的tRNA进行分析，结果表明在这些生物体中TAG和TAA编码谷氨酰胺或谷氨酸，而TGA是唯一的终止密码子。因此，人们普遍认为所有纤毛虫都使用一种非通用遗传密码，其中TGA作为唯一的终止密码子。我们已经对携带来自厚尾真核生物的肌动蛋白基因和β-微管蛋白基因的线性DNA分子进行了测序。基于与已知肌动蛋白和β-微管蛋白基因序列的同源性，这些基因显示使用TAA作为终止密码子。此外，我们对厚尾真核生物H4组蛋白基因3'末端的一部分进行了测序，并表明它也使用TAA作为终止密码子。这些数据表明，必须重新考虑纤毛虫中遗传密码变化的时间。
